1. Field of the Invention
This invention relates in general to electronic production of printing forms and in particular to the reproduction by engraving devices and scanners and particularly relates to a method and apparatus for intensifying contrast during the reproduction of masters.
2. Description of the Prior Art
In electronic printing form production using engraving devices and scanners, the masters to be produced are scanned point-by-point and line-by-line with opto-electronic scanning means and the image signal thus obtained is corrected according to the requirements of the reproduction process. The corrected image signal controls a recording element which rerecords the master on a recording medium.
Generally, the masters to be reproduced are a combination of continuous-tone images and written characters or line elements of graphic representations. Both continuous-tone images as well as written characters or line elements have contours which make reproduction difficult.
The contrast at contours particularly in fine details is already reproduced in the photographic production of a film master as compared to the original causing reduction due to the unsharpness in the film layers as well as due to enlargement or transfer copying. Also, the resolution of the opto-electronic scanning element is limited due to scattered light and due to the unsharpness of the scanning lens such that further reduction of contrast appears at contours during reproduction and the human eye interprets this as a lack of sharpness.
Thus, it is necessary to regain the reduced contrast or, respectively, the reduced image sharpness in the printing form production or of even increasing it relative to the original for editorial reasons.
For purposes of intensifying contrast or respectively, sharpness by electronic unsharp masking technique, it is already known from U.S. Pat. No. 2,691,696 to scan the surround field called unsharp masking area of the current image point with a correspondingly larger unsharp masking diaphragm in addition to scanning the current image point with an image point diaphragm so as to form a differential signal from the unsharp masking signal and the image signal and to superimpose it to a selectable amount on the image signal as a correction signal for intensifying contrast. This method which corresponds to unsharp masking in photographic reproduction technology results in an improvement of the detail contrast or, respectively, of the image sharpness at a contour or outline since a dark image change and a light image change in the direct proximity of a discontinuity of tonal value or, respectively, recorded darker and brighter than they are at some distance from the discontinuity in tonal value.
In this prior art known solution, the amplitude of the correction signal depends only on the amplitude of the tonal value or, respectively, the image signal discontinuity so that hard contours of line elements and written characters where great skips in tonal value occurs are normally overemphasized, but soft contours such as in continuous-tone images which have small skips of tonal value are only slightly corrected in such system.
In practice, however, it is a matter of particularly importance to boost the detail contrast at soft contours in continuous-tone images since written characters and line elements are more sharply reproduced anyway. So as to be able to achieve the desired effect in continuous-tone images it requires that the available corrections signal be superimposed on the image signal at full strength or full value in this prior art method. In this case, however, hard contours arise, for example, at highlights and at white edges or margins which is disruptive around the black written characters and these are particularly noticeable when, for example, black print is placed in a gray scale.
In practice, also, it would be desirable to obtain a contrast boost at soft contours or continuous-tone images having a high effective width of the correction signal but in contrast to undertake a contrast boost with a small effective width of the correction signal at hard contours of line elements and written characters such that the disruptive edges and margins would be considerably reduced.
In the prior art method, the effective width of the correction signal or, respectively, the width of the image signal intensification is rigidly prescribed by the diameter of the unsharp masking diagram utilized and can only be altered by changing the size of the diagram.
The prior art method therefore has the disadvantage that the contrast boost can be optimized in amplitude and the effective width of the correction signal only for continuous-tone regions or for line regions of the master and not for both.
It has been proposed to improve the known method by distorting amplitude limitation or by means of an asymmetrical treatment of the correction signal as well as by influencing the correction signal as a function of a switch mask as described in German LP No. 949443, German LP No. 1039842 or German LP No. 2226990, but difficulties continue in that disruptive edges or margins still occur and this is undesirable.